I have twin 525Sc's with 500 hrs. have run flawless (ok almost) for 15 years for me.
Chased a bad rev limiter a couple years ago. deleted them
in that process I replaced cap/rotor/plug wires/plugs/ nickerson rebuilt the carbs.......
getting plug wires on the plugs confidently was a PITA.

So....one engine, 100-200 rpm is dropping out randomly them coming back in. at idle, mid, and high rpm. on 10 seconds, off 10 seconds...
when on, the two engines are dead on.
I just bought a plug boot puller and will check plugs, grease boots, and reinstall (confidently with my new tool).
I am Wondering if a boot is not seated?

If not the solution, could I have a Gil exhaust water issue?
is there a way to test the Gil exhaust for a leak?
is this behavior typical of "reversion" do to water leak within exhaust manifold?
Will a plug tell me this is the case?

thanks
Ramb

When you get the plug wire issue figured out and still have the issue…..You still have the original t/bolt iv modules in the distributor ? If so, i’d replace with current new style before chasing other possibles.

one benefit of twins is swapping parts. I had swapped the TIV modules, as well as new pickups in the distributors when I was chasing the rev limiter issue.

But will look into it.

tks

Ramb

Not likely reversion with stock FT cam and at WOT. Plug wire grounding out possibly but then again doesn’t sound like it. Cracked porcelain? I’d go with SB on the module. Try swapping out with other side. Ck ohms on plug wires.

Thanks Getr
Not reversion in the typical sense due to cam LSA, but water intrusion due to manifold leak close to exhaust port?

regarding plug wires, I wasn't initially thinking of a short, but a discontinuity as I may have not seated the boot onto the plug well, resulting in intermittent connection/disconnection. I hope it is as simple as a not fully seated cable boot onto a plug. but will check for shorts as well.

Can one tell if a plug is not firing (due to open, not due to short) with a clamp on a wire or with timing light? ( ie need current draw to trigger correct?)

I doubt it’s a plug boot being intermittent and the length of time you mentioned. In regards to the Gil’s they’re certainly notorious for cracking yes and typically easy to identify under the riser. Good time to ck for sure but doubt that’s what’s causing your problem. I’d start swapping out one ign component at a time starting with the module.

Totally diff scenario but I dealt with a toro 62” 3 cylinder mower that was losing a cylinder intermittent for nearly a summer with doing all the above and started it one night after dark to move it and quickly saw a plug wire arcing. 😳

Old (auto) trick to find bad electrical (coils/ wires etc) is to take a spray bottle with water set to a fine mist....spray over your coil or ? Is suspect and the electrical leaks will be immediately apparent.

Make damn sure there are no fumes or fuel leaks as you are inducing a spark if you have an electrical leak.

Does it sound/feel like it's "missing" when RPMs drop? If not, you could watch timing marks with timing light and see if there's a big corresponding change going into and out of the lower RPM mode. There will be some change in timing simply due to RPMs changing and determining if the timing change is the cause or a symptom of RPMs changing can be tricky. That's why I said look for big changes in timing. Having another healthy eng to compare with should help a lot.

Many years ago I worked in a tune-up shop. We would do the spray bottle method on the new-at-the-time distributorless ign systems. A lot of times it would be Ok at idle, but if you power-braked the eng, it would start popping and bucking. Got some weird looks from customers at times doing this but it does work.

Being that it does it at idle too, that should make troubleshooting a lot easier.